Often the successful treatment of disorders and diseases is hampered by a lack of specificity. Pharmaceuticals which are very effective for treating a disorder in one organ or tissue may have undesirable effects in other tissues and thus limit the usefulness of the treatment. In this way, the effectiveness of drugs and therapeutic genes depends on tissue-specific delivery. One prospect for achieving tissue-specific delivery of drugs is the use of ligands which bind to specific cell types. However, the development of tissue-specific ligands for many differentiated tissues is limited by a lack of information on their cell-specific surface receptors. One solution to this problem is the selection of ligands using phage display libraries. This approach requires no prior knowledge of the target cell receptor expression and function (Barry et al. (1996) Nature Med. 2:299–305). The screening of M13 phage display peptide libraries was first employed to select cell-specific ligands in vitro, and it has also proven useful for the rapid in vivo discovery of small molecule ligands specific to various organs and tissues (Pasqualini and Ruoslahti (1996) Nature 380:364–366; Pasqualini (1999) Q. J. Nucl Med. 43:159–162; Samoylova and Smith (1999) Muscle & Nerve 22:460–466). Peptides identified via phage display screening were used to achieve selective delivery of the cytotoxic drug doxorubicin to tumors, thereby decreasing side effects from the drug's effects on untargeted organs and systems (Arap et al. (1998) Science 279:377–380). Specific gelatinase inhibitors identified from phage display peptide libraries can prevent the migration of human endothelial cells and tumor cells, thereby reducing tumor growth in mice bearing human tumors (Koivunen et al. (1999) Nat. Biotechnol. 17:768–774). Phage display screening can also be used to improve the properties of known proteins or nucleic acids. For example, the selectivity of intravenously administered adenoviral vectors was improved significantly by the insertion of phage derived peptide motifs into the H1 loop of the fiber knob protein (Reynolds et al. (1999) Gene Ther. 6:1336–1339).
Thus, peptides generated by phage display may have both therapeutic and diagnostic utility. They can be useful for the development of gene therapy vectors or drugs targeting various organs and tissues. However, while the in vivo phage display screening protocol has been successful in identifying tissue-specific ligands in mice and dogs, in vivo screening requires euthanasia and thus cannot be applied to humans. Thus, there remains a need for compositions and methods for the identification of interspecies cell-specific peptides, particularly for use in humans. Identification of interspecies cell-specific peptides by selection in multiple animal species and then determination of their affinity to human tissues would allow isolation of tissue-specific molecules that may be used as targeting ligands in gene/drug therapy protocols.
“Biosensors” have been reported in the literature, but the reported devices have low sensitivity or long response times. Decker et al. ((2000) J. Immunol. Methods 233:159–165) reported that more than 90 minutes were needed to measure phage binding by peptide fragments immobilized by biotin/streptavidin coupling. Hengerer et al. ((1999) Biotechniques 26: 956–60, 962, 964) reported binding of phage antibodies to antigen immobilized on a quartz crystal microbalance with a time constant of about 100 min. These long response times are not compatible with rapid screening and make large-scale screening unwieldy. Therefore, there remains a need for a biosensor which can rapidly detect specific proteins.